Fig. 21.15 The Red Algae reproduction

They are about 2 to 15 centimeters (1 to 6 inches) tall and are branched into many fine, threadlike segments. Spermatangia, the male sex structures, slightly resemble dense clusters of tiny grapes on slender branches of the male gametophyte thallus. Each spermatangium contains a single spermatium that functions as a nonmotile male gamete. The female sex structures, called carpogonia, are produced on the female gametophyte thallus. Each carpogonium consists of a single cell that looks something like a microscopic bottle with a long neck called a trichogyne. A single nucleus at the base of the carpogonium functions as the female gamete, or egg. Since the spermatia have no flagella, they cannot move of their own accord, but currents may carry them considerable distances. If a spermatium should brush against a trichogyne, it may become attached. The walls between the spermatium and the trichogyne then break down, the nucleus of the spermatium migrates to the egg nucleus, and the two nuclei unite, forming a zygote. Next, toward the base of a pericarp (an urn-shaped body, the outer part of which is formed by the female gametophyte thallus), the zygote begins to divide and eventually develops a cluster of clublike carposporangia. The pericarp and carposporangia combined constitute the cystocarp.

Diploid asexual spores called carpospores are produced in the carposporangia and released, to be carried away by ocean currents. When a carpospore lodges in a suitable location (e.g., a rock crevice or the hull of a ship), it usually germinates and grows into a tetrasporophyte, which closely resembles a gametophyte thallus. Tetrasporangia are formed along the branches of the tetrasporophytes. Each tetrasporangium undergoes meiosis, giving rise to four haploid tetraspores. When tetraspores germinate, they develop into male or female gametophytes, thereby completing the life cycle. The red to purplish colors of most red algae are due to the presence of varying amounts of red and blue accessory pigments called phycobilins, similar to those found in the cyanobacteria. The similarity led to the belief of some phycologists (those who specialize in the study of algae) that the red algae may have been derived from the cyanobacteria. Several other pigments, including chlorophyll a and sometimes chlorophyll d, are also present in the chloroplasts. The principal reserve food is a carbohydrate called floridean starch. A number of red algae also produce agarand other important gelatinous substances discussed in “Agar” under the section entitled.

Table 21.1.

PLAN OF THEME STUDY:

I. Homework

TASK № 1. Using lectures and textbooks, learn basic theoretical issues.

TASK № 2. Learn English and Latin names, herbaria and demonstration table following families:

Superkingdom procariote organisms - Procariota

Date: 2016-01-03; view: 1022